Mixed slurry of strong and weak graphene oxides and preparation method of mixed slurry, and composite film of strong and weak graphene oxides and preparation method of composite film

ABSTRACT

Provided are a slurry of graphene oxides with different degrees of oxidation, a composite film of graphene oxides, and a graphene heat-conducting film. The slurry of the graphene oxides comprises the graphene oxides and a solvent, and the graphene oxides include a strong graphene oxide and a weak graphene oxide, wherein the slurry comprises two graphene oxides with different degrees of oxidation, which can increase a carbon content in the graphene oxide per unit mass, so that the finally obtained graphene heat-conducting film has more carbon.

TECHNICAL FIELD

The present invention relates to the field of materials, andparticularly relates to a process for preparing a grapheneheat-conducting film from graphene oxides.

BACKGROUND ART

Graphene has a very good heat-conducting property and wide applicationprospects. It is a commonly used process for preparing a grapheneheat-conducting film by arranging and assembling a graphene oxide into agraphene oxide film and then performing high-temperature treatment. Thegraphene heat-conducting film prepared by this method is a highlyoriented heat-conducting film formed by stacking multiple layers ofgraphene, has the characteristics of being good in mechanical property,high in heat-conducting coefficient, light in weight, thin in material,good in flexibility and the like, and can solve the heat dissipationproblems in the industries of electronics, aerospace, medical treatmentand the like.

The graphene oxide used in the prior art is a graphene oxide obtained bychemical oxidation of graphite, has abundant oxygen-containingfunctional groups, and generally has a carbon content of about 50%. Suchgraphene oxide has good arrangement and assembly properties, but whenthe graphene oxide is used to prepare the graphene heat-conducting film,the heat-conducting film finally obtained per unit mass of the grapheneoxide has less carbon, which increases the cost and is not conducive toindustrial production. The content of the background art is merely atechnology known to the inventors and does not of course represent theprior art in this field.

SUMMARY OF THE INVENTION

In view of one or more of the problems in the prior art, the presentinvention provides a slurry of graphene oxides, and the slurry comprisestwo graphene oxides with different degrees of oxidization, which canincrease a carbon content in the graphene oxide per unit mass, so that afinally obtained heat-conducting film of graphene has more carbon.

Based on the above-mentioned purposes, the present invention provides amixed slurry of strong and weak graphene oxides, comprising grapheneoxides and a solvent, and the graphene oxides include a strong grapheneoxide and a weak graphene oxide.

The strong graphene oxide of the present invention refers to a grapheneoxide with a strong degree of oxidation, and the weak graphene oxiderefers to a graphene oxide with a weak degree of oxidation.

The graphene oxide is obtained by adding some substances and structuresbetween layers of graphite to strip away graphite sheet layers, and onlyto a certain degree of intercalation and oxidation can it be stripped toobtain very thin sheet layers (3 layers and below). The graphene oxideis high in degree of oxidation, large in number of oxygen-containingfunctional groups, thinner in sheet layer after striping, more orderlywhen being assembled into a film and better in assembly effect, but acarbon content of the strong graphene oxide is low, and grapheneobtained after high-temperature treatment is relatively small in amount.A carbon content of the weak graphene oxide is high, and grapheneobtained after high-temperature treatment is relatively large in amount;however, due to the weak degree of oxidation, it is only oxidized on anedge of a graphite sheet, a graphene oxide with a relatively smallnumber of sheet layers cannot be obtained by stripping, and it is alsovery difficult to prepare a graphene oxide with a weak degree ofoxidation and a small number of layers after stripping throughexperiments. The weak graphene oxide is low in degree of oxidation,small in number of oxygen-containing functional groups and large innumber of layers during stripping, if only the weak graphene oxide isassembled for film formation, a highly oriented structure is notobtained during film formation, and orderliness is not good, therebyaffecting the improvement of a heat-conducting coefficient of a grapheneheat-conducting film obtained after high-temperature treatment.

It is very difficult to control the degree of oxidation in a preparationprocess of the graphene oxide. At present, it is difficult to prepare agraphene oxide with a degree of oxidation between a strong degree ofoxidation and a weak degree of oxidation, and assembly of the grapheneoxide during film formation can be affected and the heat-conductingcoefficient of the graphene heat-conducting film prepared after finalhigh-temperature treatment is greatly affected if the degree ofoxidation of the graphene oxide slightly decreases and the sheet layeris stripped not well. Therefore, selecting a graphene oxide with asingle degree of oxidation is not compatible to obtain a grapheneheat-conducting film with a large amount of residual graphene and a goodheat-conducting property.

The present invention provides a mixed slurry of strong and weakgraphene oxides, and when orderliness of assembly for film formation isnot greatly affected, the graphene heat-conducting film afterhigh-temperature treatment is large in amount and the heat-conductingproperty of the graphene heat-conducting film is good.

According to one aspect of the present invention, a solid content of thegraphene oxide is 5-8%.

According to one aspect of the present invention, the weak grapheneoxide accounts for 5-50% of the graphene oxide by mass, preferably5-30%, further preferably 10-30%.

According to one aspect of the present invention, a carbon content ofthe strong graphene oxide is 45-55%.

Preferably, a carbon content of the weak graphene oxide is 80-90%.

The carbon content of the weak graphene oxide is selected in a range of80-90%, and because a graphene oxide with an excessively high carboncontent is large in number of layers during stripping, the orderlinesswill become poor during assembly.

According to one aspect of the present invention, the solvent includesone or a mixed solvent of two or more of water, NMP and DMF.

Different from graphene containing no oxygen-containing functionalgroups, the two graphene oxides with different degrees of oxidation aremixed in the solvent, because the two graphene oxides both containhydrophilic oxygen-containing functional groups on surfaces, and both ofthem have negative charges, they repel each other in the solvent, andare not likely to be agglomerated, and good mutual dispersion propertiesare achieved.

The present invention further provides a preparation method of a mixedslurry of strong and weak graphene oxides, specifically comprising thefollowing steps:

preparing a strong graphene oxide dispersion liquid and a weak grapheneoxide dispersion liquid respectively, mixing the strong graphene oxidedispersion liquid and the weak graphene oxide dispersion liquid, anddispersing to obtain the mixed slurry of the strong and weak grapheneoxides.

Theoretically, the strong graphene oxide and the weak graphene oxide arenot likely to be agglomerated in a solution because the functionalgroups on the surface have negative charges —COOH and thus they repeleach other. However, an agglomeration problem still occurs in actualproduction and the stability of an obtained slurry is still not idealenough. After multi-directional analysis, the inventors of the presentapplication find that agglomeration is not caused by problems such as aparticle size, specific surface and oil absorption value of the grapheneoxide, but the charge presented by the graphene oxide in the slurryaffects a system. Since the graphene oxide dispersion liquid is acidicdue to the —COOH group, it is necessary to add alkali into thedispersion liquid for neutralization during dispersing. If the twographene oxides are directly mixed and then the alkali is added, sincethe two graphene oxides have different degrees of oxidation anddifferent amounts of —COOH contained on their structures, when thealkali is added for neutralization, a reaction will be uneven, resultingin poor dispersing. By changing a process procedure, the two grapheneoxides are first dispersed well respectively, the —COOH groups on thegraphene oxides are fully neutralized, and then the two are mixed, thusavoiding the above-mentioned problem of uneven dispersing. Therefore,the two graphene oxides are first dispersed in the solvent respectively,and the —COOH of the graphene oxides is sufficiently neutralized, andthen the two are mixed, so that the two graphene oxides in a mixedliquid are not likely to be agglomerated.

According to one aspect of the present invention, a preparation methodof the strong graphene oxide dispersion liquid comprises: dispersing agraphene oxide prepared by a chemical oxidation method into the solventto obtain the strong graphene oxide dispersion liquid.

Preferably, in the chemical oxidation method, graphite reacts in astrong acid solution with an oxidizing agent to obtain the grapheneoxide.

Further preferably, the chemical oxidation method comprises a Brodiemethod, a Staudenmaier method and a Hummers method.

According to one aspect of the present invention, in the strong grapheneoxide dispersion liquid, a solid content of the strong graphene oxide is3-8%.

Preferably, in the weak graphene oxide dispersion liquid, a solidcontent of the weak graphene oxide is 7-11%.

Since the weak graphene oxide contains less oxygen-containing functionalgroups and bonds with less solvent, it is easier to obtain ahigh-concentration slurry; since the number of the layers of the weakgraphene oxide after stripping is greater than the number of the layersof the strong graphene oxide after stripping, it is possible to obtainthe weak graphene oxide dispersion liquid with a relatively high solidcontent.

According to one aspect of the present invention, a mixed liquid of thestrong graphene oxide dispersion liquid and the weak graphene oxidedispersion liquid is dispersed at a high speed, and a linear velocity ofdispersing is 50-300 m/s, preferably 100 m/s.

The present invention further provides a composite film of strong andweak graphene oxides, comprising a strong graphene oxide and a weakgraphene oxide, a content of the weak graphene oxide being 5-50%,preferably 5-30%, further preferably 10-30%.

Because a stripping degree of the strong graphene oxide is good, it canhelp the weak graphene oxide to assemble orderly; because the carboncontent of the weak graphene oxide is high, the graphene heat-conductingfilm prepared from the composite film of the graphene oxides per unitmass increases, and cost is saved. If only the strong graphene oxide isused to prepare the graphene heat-conducting film, the heat-conductingcoefficient of the finally obtained graphene heat-conducting film ishigh, but a thickness of the graphene heat-conducting film is small; ifonly the weak graphene oxide is used to prepare the grapheneheat-conducting film, or the content of the weak graphene oxide is toohigh, since it is very difficult to strip the graphene oxide with theweak degree of oxidization to obtain a graphene oxide with a smallnumber of layers (less than 10 layers), the orderliness during assemblyis not good, and the heat-conducting coefficient of the grapheneheat-conducting film after final heat treatment is low; if only agraphene oxide with an intermediate degree of oxidization is used, thedegree of oxidization thereof is lower than that of the strong grapheneoxide, and the assembly of the graphene oxide is affected due toreduction of the degree of oxidization, eventually resulting in that theheat-conducting coefficient of the graphene heat-conducting film afterheat treatment is low. Therefore, by selecting the content of the weakgraphene oxide to be 10-30%, the graphene heat-conducting film with arelatively high heat-conducting coefficient and a relatively highresidual proportion of carbon can be obtained, so that the cost can bereduced to the maximum extent.

According to one aspect of the present invention, a grammage of thecomposite film of the graphene oxides is 100-250 g/m², preferably 180g/m².

Preferably, a carbon content of the strong graphene oxide is 45-55%.

Preferably, a carbon content of the weak graphene oxide is 80-90%.

According to one aspect of the present invention, the strong grapheneoxide and the weak graphene oxide of the composite film aredirectionally arranged in plane. In-plane directional arrangement meansthat the graphene oxide is assembled into the film by hydrogen bonds,and is directionally arranged in an assembled plane, so that heat of thegraphene heat-conducting film after heat treatment can be transmitted ina direction of directional arrangement of graphene.

The present invention further provides a preparation method of acomposite film of strong and weak graphene oxides, specificallycomprising the following steps:

preparing a mixed slurry of the strong and weak graphene oxides by usingthe preparation method of the mixed slurry of the strong and weakgraphene oxides; and

defoaming, coating and drying the slurry to form the composite film ofthe strong and weak graphene oxides.

According to one aspect of the present invention, a continuous on-linedefoaming machine is used for defoaming, and a rotating speed is2000-3000 r/min.

Preferably, a feeding valve of the defoaming machine has an openingdegree of 25-35%, preferably 30%.

Preferably, a vacuum negative pressure in the defoaming machine is400-600 Pa, preferably 500 Pa.

According to one aspect of the present invention, a method of coatingcomprises knife coating and slot die coating, preferably knife coating,and further preferably, a coating machine is used for performing knifecoating.

In a method of slot die coating, the slurry at a certain flow rateenters a die head inner cavity from an extruding head feeding port, andstable pressure is formed; finally the slurry is sprayed out of a diehead slot outlet to be coated on a base material.

Knife coating is a coating mode in which a scraping knife is adopted forperforming coating so as to make a thick coating film.

Preferably, a thickness of a wet film formed by coating is 2-5 mm,preferably 3 mm. Since the carbon content of the weak graphene oxide ishigh, the carbon residue value of the final heat-conducting film can beincreased, so that a thicker graphene heat-conducting film can beobtained in the case of the same thickness of graphene oxide coating. Ifit is needed to prepare a graphene heat-conducting film of the samethickness, by using the method of the present invention, the thicknessof the wet film formed by coating can be made smaller, and filmproduction cost can be greatly reduced and efficiency can be improved.

Preferably, a speed of coating is 1-2 m/min, preferably 1.2 m/min.

According to one aspect of the present invention, a temperature ofdrying is 50-100° C., and time of drying is 0.5-1.5 h.

The present invention further provides a graphene heat-conducting film,and a heat-conducting coefficient of the graphene heat-conducting filmis greater than 1000 W/m·K.

According to one aspect of the present invention, a density of thegraphene heat-conducting film is 1.9-2.1 g/cm³.

Preferably, a thickness of the graphene heat-conducting film is 30-100The present invention further provides a preparation method of agraphene heat-conducting film, comprising the following steps:

preparing a composite film of strong and weak graphene oxides by usingthe preparation method of the composite film of the strong and weakgraphene oxides;

performing high-temperature treatment on the composite film; and

compacting the composite film after high-temperature treatment to obtainthe graphene heat-conducting film.

Because the carbon content of the composite film of the graphene oxidesis high, the amount of the graphene heat-conducting film afterhigh-temperature treatment is large, which can greatly reduce the filmproduction cost and improve the efficiency.

According to one aspect of the present invention, a method forperforming high-temperature treatment on the composite film comprises:heating the composite film from a room temperature to 1000-3000° C., andperforming treatment for 5-15 h.

Preferably, a heating rate is 0.2-5° C./min.

Further preferably, the heating rate is 1-1.5° C./min at 100° C. orless, the heating rate is 0.2-0.5° C./min at more than 100° C. and 300°C. or less, and the heating rate is 3-5° C./min at more than 300° C.

According to one aspect of the present invention, a compacting pressureis 10-30 MPa.

The present invention has the beneficial effects.

In the present invention, the graphene heat-conducting film is preparedby mixing the two graphene oxides with different degrees, the grapheneoxide with the strong degree of oxidization has abundant functionalgroups and good assembly orderliness, and the graphene oxide with theweak degree of oxidization has a high carbon content, so that theprepared graphene heat-conducting film is thicker and the proportion ofthe final residual carbon is increased, which can improve the productionefficiency and effectively reduce cost. Comparing a slurry of a grapheneoxide with a strong degree of oxidization in the same concentration withthe mixed slurry of the strong and weak graphene oxides of the presentinvention, it is only needed to form the wet film of 2 mm by coating theslurry of the present invention and then a 32 μm grapheneheat-conducting film can be obtained, while a 31 μm grapheneheat-conducting film can be obtained when it is needed to form a wetfilm of 4.5 mm by coating the slurry of the graphene oxide with thestrong degree of oxidization; when the wet film of 4.5 mm is formed bycoating the slurry of the graphene oxide with the strong degree ofoxidization and the mixed slurry of the strong and weak graphene oxidesrespectively, the 31 μm graphene heat-conducting film is finallyobtained from the slurry of the graphene oxide with the strong degree ofoxidization, and a 72 μm graphene heat-conducting film is finallyobtained from the mixed slurry of the strong and weak graphene oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are used to provide a further understanding of the presentinvention and constitute a part of this description, are used toillustrate the present invention together with examples of the presentinvention, and do not constitute a limitation to the present invention.In the drawings:

FIG. 1 is an SEM view of a graphene oxide film of Example 1;

FIG. 2 is an SEM view of a graphene oxide film of Example 2;

FIG. 3 is an SEM view of a graphene oxide film of Example 3;

FIG. 4 is an SEM view of a graphene oxide film of Comparative Example 4;and

FIG. 5 is an SEM view of a graphene oxide film of Comparative Example 5.

DETAILED DESCRIPTION

In the following, only certain exemplary examples are briefly described.As will be recognized by those skilled in the art, the describedexamples can be modified in various ways without departing from thespirit or scope of the present invention. Accordingly, the drawings anddescription are regarded as illustrative in nature, and not restrictive.

The preferred examples of the present invention will be described belowwith reference to the drawings, and it should be understood that thepreferred examples described herein are only used to illustrate andexplain the present invention and not used to limit the presentinvention.

As a first embodiment of the present invention, a mixed slurry of strongand weak graphene oxides is shown and comprises graphene oxides and asolvent, and the graphene oxides include a strong graphene oxide and aweak graphene oxide. A solid content of the graphene oxide is 5-8%, forexample: 5%, 5.2%, 5.4%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8%, 7%, 7.2%,7.5%, 7.8%, 8%, etc. The weak graphene oxide accounts for 5-50% of thegraphene oxides by mass, for example: 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%,25%, 30%, 35%, 40%, 42%, 45%, 48%, 49%, 50%, etc. A carbon content ofthe strong graphene oxide is 45-55%, for example: 45%, 46%, 47%, 48%,49%, 50%, 51%, 52%, 53%, 54%, 55%, etc. A carbon content of the the weakgraphene oxide is 80-90%, for example: 80%, 81%, 82%, 83%, 84%, 85%,86%, 87%, 88%, 89%, 90%, etc. As a preferred embodiment, the weakgraphene oxide accounts for 5-30% of the graphene oxides by mass, forexample: 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%,19%, 20%, 22%, 25%, 28%, 29%, 30%, etc. As an optimal embodiment, theweak graphene oxide accounts for 10-30% of the graphene oxides by mass,for example: 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%,22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, etc. The carbon content ofthe weak graphene oxide is selected in a range of 80-90%, because agraphene oxide with an excessively high carbon content has a high numberof layers during stripping, and orderliness becomes poor duringassembling. The solvent includes one or a mixed solvent of two or moreof water, NMP and DMF.

The strong graphene oxide of the present invention refers to a grapheneoxide with a strong degree of oxidation, and the weak graphene oxiderefers to a graphene oxide with a weak degree of oxidation.

The graphene oxide is obtained by adding some substances and structuresbetween layers of graphite to strip away graphite sheet layers, and onlyto a certain degree of intercalation and oxidation can it be stripped toobtain very thin sheet layers (3 layers and below). The graphene oxideis high in degree of oxidation, large in number of oxygen-containingfunctional groups, thinner in sheet layer after striping, more orderlywhen being assembled into a film and better in assembly effect, but acarbon content of the strong graphene oxide is low, and grapheneobtained after high-temperature treatment is relatively small in amount.A carbon content of the weak graphene oxide is high, and grapheneobtained after high-temperature treatment is relatively large in amount;however, due to the weak degree of oxidation, it is only oxidized on anedge of a graphite sheet, a graphene oxide with a relatively smallnumber of sheet layers cannot be obtained by stripping, and it is alsovery difficult to prepare a graphene oxide with a weak degree ofoxidation and a small number of layers after stripping throughexperiments. The weak graphene oxide is low in degree of oxidation,small in number of oxygen-containing functional groups and large innumber of layers during stripping, if only the weak graphene oxide isassembled for film formation, a highly oriented structure is notobtained during film formation, and orderliness is not good, therebyaffecting the improvement of a heat-conducting coefficient of a grapheneheat-conducting film obtained after high-temperature treatment.

It is very difficult to control the degree of oxidation in a preparationprocess of the graphene oxide. At present, it is difficult to prepare agraphene oxide with a degree of oxidation between a strong degree ofoxidation and a weak degree of oxidation, and assembly of the grapheneoxide during film formation can be affected and the heat-conductingcoefficient of the graphene heat-conducting film prepared after finalhigh-temperature treatment is greatly affected if the degree ofoxidation of the graphene oxide slightly decreases. Therefore, selectinga graphene oxide with a single degree of oxidation is not compatible toobtain a graphene heat-conducting film with a large amount of residualgraphene and a good heat-conducting property.

As a second embodiment of the present invention, a preparation method ofa mixed slurry of strong and weak graphene oxides is shown, andspecifically comprises the following steps:

preparing a strong graphene oxide dispersion liquid and a weak grapheneoxide dispersion liquid respectively, mixing the strong graphene oxidedispersion liquid and the weak graphene oxide dispersion liquid, anddispersing to obtain the mixed slurry of the strong and weak grapheneoxides.

Theoretically, the strong graphene oxide and the weak graphene oxide arenot likely to be agglomerated in a solution because functional groups ona surface have negative charges —COOH and thus they repel each other.However, an agglomeration problem still occurs in actual production andthe stability of an obtained slurry is still not ideal enough. Aftermulti-directional analysis, the inventors of the present applicationfind that agglomeration is not caused by problems such as a particlesize, specific surface and oil absorption value of the graphene oxide,but the charge presented by the graphene oxide in the slurry affects asystem. Since the graphene oxide dispersion liquid is acidic due to the—COOH group, it is necessary to add alkali into the dispersion liquidfor neutralization during dispersing. If the two graphene oxides aredirectly mixed and then the alkali is added, since the two grapheneoxides have different degrees of oxidation and different amounts of—COOH contained on their structures, when the alkali is added forneutralization, a reaction will be uneven, resulting in poor dispersing.By changing a process procedure, the two graphene oxides are firstdispersed well respectively, the —COOH groups on the graphene oxides arefully neutralized, and then the two are mixed, thus avoiding theabove-mentioned problem of uneven dispersing. Therefore, the twographene oxides are first dispersed in the solvent respectively, and the—COOH of the graphene oxides is sufficiently neutralized, and then thetwo are mixed, so that the two graphene oxides in a mixed liquid are notlikely to be agglomerated.

A preparation method of the strong graphene oxide dispersion liquidcomprises: dispersing a graphene oxide prepared by a chemical oxidationmethod into the solvent to obtain the strong graphene oxide dispersionliquid. In the chemical oxidation method, graphite reacts in a strongacid solution with an oxidizing agent to obtain the graphene oxide. Thechemical oxidation method comprises a Brodie method, a Staudenmaiermethod and a Hummers method.

In the strong graphene oxide dispersion liquid, a solid content of thestrong graphene oxide is 3-8%, for example: 3%, 4%, 5%, 6%, 7%, 8%, etc.In the weak graphene oxide dispersion liquid, a solid content of theweak graphene oxide is 7-11%, for example: 7%, 8%, 9%, 10%, 11%, etc.Since the weak graphene oxide contains less oxygen-containing functionalgroups and bonds with less solvent, it is easier to obtain ahigh-concentration slurry; since the number of the layers of the weakgraphene oxide after stripping is greater than the number of the layersof the strong graphene oxide after stripping, it is possible to obtainthe weak graphene oxide dispersion liquid with a relatively high solidcontent.

A mixed liquid of the strong graphene oxide dispersion liquid and theweak graphene oxide dispersion liquid is dispersed at a high speed, anda linear velocity of dispersing is 50-300 m/s, for example: 50 m/s, 60m/s, 70 m/s, 80 m/s, 90 m/s, 100 m/s, 120 m/s, 150 m/s, 180 m/s, 200m/s, 220 m/s, 240 m/s, 250 m/s, 280 m/s, 290 m/s, 300 m/s, etc. As apreferred embodiment, the linear velocity of dispersing is 100 m/s.

As a third embodiment of the present invention, a composite film ofstrong and weak graphene oxides is shown and comprises a strong grapheneoxide and a weak graphene oxide, a content of the weak graphene oxide is5-50%, for example: 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%,40%, 42%, 45%, 48%, 49%, 50%, etc. As a preferred embodiment, thecontent of the weak graphene oxide is 5-30%, for example: 5%, 6%, 7%,8%, 9%, 10%, 12%, 15%, 18%, 19%, 20%, 21%, 24%, 25%, 28%, 29%, 30%, etc.As an optimal embodiment, the content of the weak graphene oxide is10-30%, for example: 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%,20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, etc.

Because a stripping degree of the strong graphene oxide is good, it canhelp the weak graphene oxide to assemble orderly; because the carboncontent of the weak graphene oxide is high, the graphene heat-conductingfilm prepared from the composite film of the graphene oxides per unitmass increases, and cost is saved. If only the strong graphene oxide isused to prepare the graphene heat-conducting film, the heat-conductingcoefficient of the finally obtained graphene heat-conducting film ishigh, but a thickness of the graphene heat-conducting film is small; ifonly the weak graphene oxide is used to prepare the grapheneheat-conducting film, or the content of the weak graphene oxide is toohigh, since it is very difficult to strip the graphene oxide with theweak degree of oxidization to obtain a graphene oxide with a smallnumber of layers (less than 10 layers), the orderliness during assemblyis not good, and the heat-conducting coefficient of the grapheneheat-conducting film after final heat treatment is low; if only agraphene oxide with an intermediate degree of oxidization is used, thedegree of oxidization thereof is lower than that of the strong grapheneoxide, and the assembly of the graphene oxide is affected due toreduction of the degree of oxidization, eventually resulting in that theheat-conducting coefficient of the graphene heat-conducting film afterheat treatment is low. Therefore, by selecting the content of the weakgraphene oxide to be 5-50%, the graphene heat-conducting film with arelatively high heat-conducting coefficient and a relatively highresidual proportion of carbon can be obtained, so that the cost can bereduced to the maximum extent.

A grammage of the composite film of the graphene oxides is 100-250 g/m²,for example: 100 g/m², 110 g/m², 120 g/m², 140 g/m², 150 g/m², 180 g/m²,200 g/m², 220 g/m², 230 g/m², 240 g/m², 250 g/m², etc. As a preferredembodiment, the grammage of the composite film of the graphene oxides is180 g/m². A carbon content of the strong graphene oxide is 45-55%, forexample: 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, etc. Acarbon content of the weak graphene oxide is 80-90%, for example: 80%,81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, etc. The stronggraphene oxide and the weak graphene oxide of the composite film of thegraphene oxides are directionally arranged in plane. In-planedirectional arrangement means that the graphene oxide is assembled intothe film by hydrogen bonds, and is directionally arranged in anassembled plane, so that heat of the graphene heat-conducting film afterheat treatment can be transmitted in a direction of directionalarrangement of graphene.

As a fourth embodiment of the present invention, a preparation method ofa composite film of strong and weak graphene oxides is shown andspecifically comprises the following steps:

preparing a mixed slurry of the strong and weak graphene oxides by usingthe method in the second embodiment; and

defoaming, coating and drying the slurry to form the composite film ofthe strong and weak graphene oxides.

A continuous on-line defoaming machine is used for defoaming, and arotating speed is 2000-3000 r/min, for example: 2000 r/min, 2100 r/min,2200 r/min, 2300 r/min, 2400 r/min, 2500 r/min, 2600 r/min, 2700 r/min,2800 r/min, 2900 r/min, 3000 r/min, etc. A feeding valve of thedefoaming machine has an opening degree of 25-35%, for example: 25%,26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, etc. As a preferredembodiment, the feeding valve of the defoaming machine has an openingdegree of 30%. A vacuum negative pressure in the defoaming machine is400-600 Pa, for example: 400 Pa, 420 Pa, 440 Pa, 450 Pa, 480 Pa, 490 Pa,500 Pa, 520 Pa, 550 Pa, 580 Pa, 590 Pa, 600 Pa, etc. As a preferredembodiment, the vacuum negative pressure in the defoaming machine is 500Pa. Methods of coating include slot die coating and knife coating. In amethod of slot die coating, the slurry at a certain flow rate enters adie head inner cavity from an extruding head feeding port, and stablepressure is formed; finally the slurry is sprayed out of a die head slotoutlet to be coated on a base material. Knife coating is a coating modein which a scraping knife is adopted for performing coating so as tomake a thick coating film. The mode of knife coating is preferablyadopted and further preferably, a coating machine is adopted forperforming knife coating. A thickness of a wet film formed by coating is2-5 mm, for example: 2 mm, 2.2 mm, 2.5 mm, 2.8 mm, 3 mm, 3.2 mm, 3.5 mm,3.8 mm, 4 mm, 4.2 mm, 4.5 mm, 4.8 mm, 5 mm, etc. As a preferredembodiment, the thickness of the wet film formed by coating is 3 mm.Since the carbon content of the weak graphene oxide is high, the carbonresidue value of the final heat-conducting film can be increased, sothat a thicker graphene heat-conducting film can be obtained in the caseof the same thickness of graphene oxide coating. If it is needed toprepare a graphene heat-conducting film of the same thickness, by usingthe method of the present invention, the thickness of the wet filmformed by coating can be made smaller, and film production cost can begreatly reduced and efficiency can be improved. A speed of coating is1-2 m/min, for example: 1 m/min, 1.1 m/min, 1.2 m/min, 1.3 m/min, 1.4m/min, 1.5 m/min, 1.6 m/min, 1.7 m/min, 1.8 m/min, 1.9 m/min, 2 m/min,etc. As a preferred embodiment, the speed of coating is 1.2 m/min. Atemperature of drying is 50-100° C., for example: 50° C., 55° C., 60°C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 100° C.,etc.; time of drying is 0.5-1.5 h, for example: 0.5 h, 0.6 h, 0.7 h, 0.8h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h, 1.5 h, etc.

As a fifth embodiment of the present invention, a grapheneheat-conducting film is shown, and the heat-conducting coefficient ofthe graphene heat-conducting film is greater than 1000 W/m·K, forexample: 1100 W/m·K, 1200 W/m·K, 1300 W/m·K, 1400 W/m·K, 1500 W/m·K,1600 W/m·K, 1700 W/m·K, 1800 W/m·K, etc. A density of the grapheneheat-conducting film is 1.9-2.1 g/cm³, for example: 1.9 g/cm³, 1.92g/cm³, 1.95 g/cm³, 1.98 g/cm³, 2.0 g/cm³, 2.02 g/cm³, 2.05 g/cm³, 2.08g/cm³, 2.1 g/cm³, etc. A thickness of the graphene heat-conducting filmis 30-100 μm, for example: 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, etc.

As a sixth embodiment of the present invention, a preparation method ofa graphene heat-conducting film is shown and comprises the followingsteps:

preparing a composite film of strong and weak graphene oxides by usingthe method in the fourth embodiment;

performing high-temperature treatment on the composite film; and

compacting the composite film after high-temperature treatment to obtainthe graphene heat-conducting film.

Because the carbon content of the composite film of the graphene oxidesis high, the amount of the graphene heat-conducting film afterhigh-temperature treatment is relatively large, which can greatly reducethe film production cost and improve the efficiency.

A method of performing high-temperature treatment on the composite filmcomprises: heating the composite film from a room temperature to1000-3000° C., for example: 1000° C., 1200° C., 1500° C., 1800° C.,2000° C., 2200° C., 2500° C., 2800° C., 3000° C., etc.; and performingtreatment for 5-15 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h,12 h, 13 h, 14 h, 15 h, etc. A heating rate is 0.2-5° C./min, forexample: 0.2° C./min, 0.3° C./min, 0.4° C./min, 0.5° C./min, 0.8°C./min, 1° C./min, 1.5° C./min, 2° C./min, 2.5° C./min, 3° C./min, 3.5°C./min, 4° C./min, 4.5° C./min, 4.8° C./min, 4.9° C./min, 5° C./min,etc. As a preferred embodiment, at 100° C. or less, the heating rate is1-1.5° C./min, for example: 1° C./min, 1.1° C./min, 1.2° C./min, 1.3°C./min, 1.4° C./min, 1.5° C./min, etc.; at more than 100° C. and 300° C.or less, the heating rate is 0.2-0.5° C./min, for example: 0.2° C./min,0.3° C./min, 0.4° C./min, 0.5° C./min, etc.; at more than 300° C., theheating rate is 3-5° C./min, for example: 3° C./min, 3.2° C./min, 3.5°C./min, 3.8° C./min, 4° C./min, 4.2° C./min, 4.5° C./min, 4.8° C./min,5° C./min, etc. A compacting pressure is 10-30 MPa, for example: 10 MPa,11 MPa, 12 MPa, 14 MPa, 15 MPa, 18 MPa, 20 MPa, 22 MPa, 24 MPa, 25 MPa,28 MPa, 29 MPa, 30 MPa, etc.

The advantages of the present invention are illustrated below byexamples and comparative examples. In the following examples, a stronggraphene oxide refers to a graphene oxide having a carbon content of45-55%; a weak graphene oxide refers to a graphene oxide with a carboncontent of 80-90%.

Example 1A

This example shows a preparation process of a mixed slurry of strong andweak graphene oxides, which specifically comprises the following steps.

Step (1): preparing a strong graphene oxide dispersion liquid, wherein 1kg of strong graphene oxide is dispersed in 19 kg of deionized water toobtain the strong graphene oxide dispersion liquid with a solid contentof 5%, with a total of 20 kg.

Step (2): preparing a weak graphene oxide dispersion liquid, wherein 0.2kg of weak graphene oxide is dispersed in 1.8 kg of deionized water toobtain the weak graphene oxide dispersion liquid with a solid content of10%, with a total of 2 kg.

Step (3): mixing the dispersion liquid obtained in step (1) and step(2), and dispersing at a high speed and at a linear velocity of 70 m/sto obtain a graphene oxide slurry with a solid content of 5.45%, theweak graphene oxide accounting for 16.7% of total solids by mass.

Example 1B

This example shows a preparation process for preparing a composite filmof strong and weak graphene oxides by using the process of Example 1A,which specifically comprises the following steps.

Step (1): preparing a mixed slurry of graphene oxides by using theprocess of Example 1A.

Step (2): using a continuous on-line defoaming machine for defoaming theslurry in step (1), a rotating speed of the defoaming machine being 2400r/min, a feeding valve having an opening degree of 30%, and a vacuumnegative pressure being 500 Pa.

Step (3): using a coating machine for knife coating of the slurry afterbeing defoamed in step (2), a thickness of a wet film formed by coatingbeing 3 mm, and a speed of continuous coating being 1.2 m/min.

Step (4): drying the wet film after coating in step (3), a 16-sectiondrying oven with a drying path length of 64 m being used for drying, anda temperature of each section of the drying oven being 60° C., 63° C.,65° C., 70° C., 75° C., 78° C., 80° C., 85° C., 85° C., 85° C., 85° C.,85° C., 78° C., 75° C., 70° C. and 60° C. respectively, so as to obtaina composite film of the strong and weak graphene oxides with a grammageof 180 g/m². It can be seen from FIG. 1 that using a small amount ofweak graphene oxide does not affect a stacking effect when the grapheneoxide is assembled, and the graphene oxide is stacked well.

Example 1C

This example shows a preparation process for preparing a grapheneheat-conducting film by using the process of Example 1B, whichspecifically comprises the following steps.

Step (1): preparing a composite film of graphene oxides by using theprocess of Example 1B. Step (2): performing heat treatment on thecomposite film, heating to 100° C. at a rate of 1.5° C./min, thenheating to 300° C. at a rate of 0.2° C./min, then heating to 3000° C. ata rate of 5° C./min, performing treatment for 1 h, and removingoxygen-containing functional groups of the composite film to obtain agraphene foam film with a thickness of 90 μm.

Step (3): increasing a density of the graphene foam film obtained instep (2) by compacting to obtain a graphene heat-conducting film havinga density of 2.1 g/cm³, a heat-conducting coefficient of 1560 W/m·K anda thickness of 49 μm.

Example 2A

This example shows a preparation process of a mixed slurry of strong andweak graphene oxides, which specifically comprises the following steps.

Step (1): preparing a strong graphene oxide dispersion liquid, wherein0.9 kg of strong graphene oxide is dispersed in 19.1 kg of deionizedwater to obtain the strong graphene oxide dispersion liquid with a solidcontent of 4.5%, with a total of 20 kg.

Step (2): preparing a weak graphene oxide dispersion liquid, wherein0.35 kg of weak graphene oxide is dispersed in 4.65 kg of deionizedwater to obtain the weak graphene oxide dispersion liquid with a solidcontent of 7%, with a total of 5 kg.

Step (3): mixing the dispersion liquid obtained in step (1) and step(2), and dispersing at a high speed and at a linear velocity of 100 m/sto obtain a graphene oxide slurry with a solid content of 5%, the weakgraphene oxide accounting for 28% of total solids by mass.

Example 2B

This example shows a preparation process for preparing a composite filmof strong and weak graphene oxides by using the process of Example 2A,which specifically comprises the following steps.

Step (1): preparing a mixed slurry of graphene oxides by using theprocess of Example 2A.

Step (2): using a continuous on-line defoaming machine for defoaming theslurry in step (1), a rotating speed of the defoaming machine being 3000r/min, a feeding valve having an opening degree of 28%, and a vacuumnegative pressure being 500 Pa.

Step (3): using a coating machine for knife coating of the slurry afterbeing defoamed in step (2), a thickness of a wet film formed by coatingbeing 2 mm, and a speed of continuous coating being 2 m/min.

Step (4): drying the wet film after coating in step (3), a 16-sectiondrying oven with a drying path length of 64 m being used for drying, anda temperature of each section of the drying oven being 55° C., 60° C.,63° C., 65° C., 70° C., 75° C., 78° C., 80° C., 80° C., 80° C., 80° C.,80° C., 78° C., 75° C., 70° C. and 60° C. respectively, so as to obtaina composite film of the strong and weak graphene oxides with a grammageof 110 g/m². It can be seen from FIG. 2 that using a small amount ofweak graphene oxide does not affect a stacking effect when the grapheneoxide is assembled, and the graphene oxide is stacked well.

Example 2C

This example shows a preparation process for preparing a grapheneheat-conducting film by using the process of Example 2B, whichspecifically comprises the following steps.

Step (1): preparing a composite film of graphene oxides by using theprocess of Example 2B.

Step (2): performing heat treatment on the composite film, heating to100° C. at a rate of 1° C./min, then heating to 300° C. at a rate of0.5° C./min, then heating to 2800° C. at a rate of 3° C./min, performingtreatment for 1 h, and removing oxygen-containing functional groups ofthe composite film to obtain a graphene foam film with a thickness of 60μm.

Step (3): increasing a density of the graphene foam film obtained instep (2) by compacting to obtain a graphene heat-conducting film havinga density of 2.0 g/cm³, a heat-conducting coefficient of 1232 W/m·K anda thickness of 33 μm.

Example 3A

This example shows a preparation process of a mixed slurry of strong andweak graphene oxides, which specifically comprises the following steps.

Step (1): preparing a strong graphene oxide dispersion liquid, wherein1.1 kg of strong graphene oxide is dispersed in 18.9 kg of deionizedwater to obtain the strong graphene oxide dispersion liquid with a solidcontent of 5.5%, with a total of 20 kg.

Step (2): preparing a weak graphene oxide dispersion liquid, wherein0.96 kg of weak graphene oxide is dispersed in 11.04 kg of deionizedwater to obtain the weak graphene oxide dispersion liquid with a solidcontent of 8%, with a total of 12 kg.

Step (3): mixing the dispersion liquid obtained in step (1) and step(2), and dispersing at a high speed and at a linear velocity of 300 m/sto obtain a graphene oxide slurry with a solid content of 6.44%, theweak graphene oxide accounting for 46.6% of total solids by mass.

Example 3B

This example shows a preparation process for preparing a composite filmof strong and weak graphene oxides by using the process of Example 3A,which specifically comprises the following steps.

Step (1): preparing a mixed slurry of graphene oxides by using theprocess of Example 3A.

Step (2): using a continuous on-line defoaming machine for defoaming theslurry in step (1), a rotating speed of the defoaming machine being 2800r/min, a feeding valve having an opening degree of 30%, and a vacuumnegative pressure being 500 Pa.

Step (3): using a coating machine for knife coating of the slurry afterbeing defoamed in step (2), a thickness of a wet film formed by coatingbeing 3.5 mm, and a speed of continuous coating being 1 m/min.

Step (4): drying the wet film after coating in step (3), a 16-sectiondrying oven with a drying path length of 64 m being used for drying, anda temperature of each section of the drying oven being 60° C., 63° C.,65° C., 75° C., 80° C., 85° C., 90° C., 95° C., 95° C., 95° C., 95° C.,95° C., 85° C., 75° C., 70° C. and 60° C. respectively, so as to obtaina composite film of the strong and weak graphene oxides with a grammageof 248 g/m². It can be seen from FIG. 3 that using a small amount ofweak graphene oxide does not affect a stacking effect when the grapheneoxide is assembled, and the graphene oxide is stacked well.

Example 3C

This example shows a preparation process for preparing a grapheneheat-conducting film by using the process of Example 3B, whichspecifically comprises the following steps.

Step (1): preparing a composite film of graphene oxides by using theprocess of Example 3B.

Step (2): performing heat treatment on the composite film, heating to100° C. at a rate of 1° C./min, then heating to 300° C. at a rate of0.2° C./min, then heating to 2600° C. at a rate of 4° C./min, performingtreatment for 1 h, and removing oxygen-containing functional groups ofthe composite film to obtain a graphene foam film with a thickness of150 μm.

Step (3): increasing a density of the graphene foam film obtained instep (2) by compacting to obtain a graphene heat-conducting film havinga density of 1.95 g/cm³, a heat-conducting coefficient of 1120 W/m·K anda thickness of 85 μm.

Comparative Example 4A

This comparative example shows a preparation process of a slurry of agraphene oxide with a strong degree of oxidization, which specificallycomprises the following steps.

0.6 kg of strong graphene oxide is dispersed in 19.4 kg of deionizedwater to obtain a strong graphene oxide slurry with a solid content of3%, with a total of 20 kg.

Comparative Example 4B

This comparative example shows a preparation process for preparing agraphene oxide film by using the process of Comparative Example 4A,which specifically comprises the following steps.

Step (1): preparing a graphene oxide slurry by using the process ofComparative Example 4A.

Step (2): using a continuous on-line defoaming machine for defoaming theslurry in step (1), a rotating speed of the defoaming machine being 2800r/min, a feeding valve having an opening degree of 30%, and a vacuumnegative pressure being 500 Pa.

Step (3): using a coating machine for knife coating of the slurry afterbeing defoamed in step (2), a thickness of a wet film formed by coatingbeing 4.5 mm, and a speed of continuous coating being 0.46 m/min.

Step (4): drying the wet film after coating in step (3), a 16-sectiondrying oven with a drying path length of 64 m being used for drying, anda temperature of each section of the drying oven being 60° C., 63° C.,65° C., 75° C., 80° C., 80° C., 80° C., 80° C., 80° C., 80° C., 80° C.,80° C., 80° C., 75° C., 70° C. and 60° C. respectively, so as to obtaina graphene oxide film with a grammage of 135 g/m². It can be seen fromFIG. 4 that by only using a strong graphene oxide, the graphene oxide isstacked well.

Comparative Example 4C

This comparative example shows a preparation process for preparing agraphene heat-conducting film by using the process of ComparativeExample 4B, which specifically comprises the following steps.

Step (1): preparing a graphene oxide film by using the process ofComparative Example 4B.

Step (2): performing heat treatment on the graphene oxide film, heatingto 100° C. at a rate of 1° C./min, then heating to 300° C. at a rate of0.2° C./min, then heating to 2850° C. at a rate of 4° C./min, performingtreatment for 1 h, and removing oxygen-containing functional groups ofthe graphene oxide film to obtain a graphene foam film with a thicknessof 106 μm.

Step (3): increasing a density of the graphene foam film obtained instep (2) by compacting to obtain a graphene heat-conducting film havinga density of 2.0 g/cm³, a heat-conducting coefficient of 1605 W/m·K anda thickness of 31 μm.

The 31 μm graphene heat-conducting film obtained in Comparative Example4 obtained and the 33 μm graphene heat-conducting film obtained inExample 2 are substantially close in thickness, but when coating,Comparative Example 4 requires coating a thicker film, and the speed ofcoating is significantly reduced and the coating cost is high. Forobtaining the graphene heat-conducting film with the same thickness,Comparative Example 4 needs a higher cost. By comparing Example 1 withComparative Example 4, Example 1 has a relatively small effect on theheat-conducting coefficient by adding little weak graphene oxide.

Comparative Example 5A

This comparative example shows a preparation process of a slurry of agraphene oxide with a weak degree of oxidization, which specificallycomprises the following steps.

1.4 kg of weak graphene oxide is dispersed in 18.6 kg of deionized waterto obtain the weak graphene oxide slurry with a solid content of 7%,with a total of 20 kg.

Comparative Example 5B

This comparative example shows a preparation process for preparing agraphene oxide film by using the process of Comparative Example 5A,which specifically comprises the following steps.

Step (1): preparing a graphene oxide slurry by using the process ofComparative Example 5A.

Step (2): using a continuous on-line defoaming machine for defoaming theslurry in step (1), a rotating speed of the defoaming machine being 2800r/min, a feeding valve having an opening degree of 30%, and a vacuumnegative pressure being 500 Pa.

Step (3): using a coating machine for knife coating of the slurry afterbeing defoamed in step (2), a thickness of a wet film formed by coatingbeing 2 mm, and a speed of continuous coating being 2.3 m/min.

Step (4): drying the wet film after coating in step (3), a 16-sectiondrying oven with a drying path length of 64 m being used for drying, anda temperature of each section of the drying oven being 60° C., 63° C.,65° C., 75° C., 80° C., 85° C., 90° C., 95° C., 95° C., 95° C., 95° C.,95° C., 85° C., 75° C., 70° C. and 60° C. respectively, so as to obtaina graphene oxide film with a grammage of 140 g/m². It can be seen fromFIG. 5 that by only using a weak graphene oxide to prepare the grapheneoxide film, since the weak graphene oxide contains a small number ofoxygen-containing functional groups, a number of stripped layers issmall, and a stacking effect is not good in assembling.

Comparative Example 5C

This comparative example shows a preparation process for preparing agraphene heat-conducting film by using the process of ComparativeExample 5B, which specifically comprises the following steps.

Step (1): preparing a graphene oxide film by using the process ofComparative Example 5B.

Step (2): performing heat treatment on the graphene oxide film, heatingto 100° C. at a rate of 1° C./min, then heating to 300° C. at a rate of0.2° C./min, then heating to 2800° C. at a rate of 4° C./min, performingtreatment for 1 h, and removing oxygen-containing functional groups ofthe graphene oxide film to obtain a graphene foam film with a thicknessof 80 μm.

Step (3): increasing a density of the graphene foam film obtained instep (2) by compacting to obtain a graphene heat-conducting film havinga density of 2.0 g/cm³, a heat-conducting coefficient of 736 W/m·K and athickness of 56 μm.

Since an assembly effect of the graphene oxide film in ComparativeExample 5B is not good, the heat-conducting coefficient of the grapheneheat-conducting film after final high-temperature treatment is low.

Finally, it should be noted that: what is described above is onlypreferred examples of the present invention, and is not used to limitthe present invention, although the present invention has been describedin detail with reference to the foregoing examples, those skilled in theart can still modify the technical solution described by each of theforegoing examples or perform equivalent replacement on part oftechnical characteristics therein. Any modification, equivalentreplacement, improvement, etc. made within the spirit and principle ofthe present invention shall be included within the protection scope ofthe present invention.

The invention claimed is:
 1. The preparation method of a grapheneheat-conducting film, comprising the following steps: preparing a firstdispersion liquid containing a strong graphene oxide and a seconddispersion liquid containing a weak graphene oxide, wherein the stronggraphene oxide has a carbon content of 45-55% and the weak grapheneoxide has a carbon content of 80-90%; mixing the first dispersion liquidand the second dispersion liquid to obtain a slurry; defoaming theslurry, coating the slurry on a surface to form a wet film, and dryingthe wet film to obtain a composite film; performing high-temperaturetreatment to the composite film; and compacting the high-temperaturetreated composite film to obtain the graphene heat-conducting film. 2.The preparation method according to claim 1, further comprising reactinga graphite in a strong acid solution containing an oxide agent to obtainthe strong graphene oxide.
 3. The preparation method according to claim1, wherein a solid content of the strong graphene oxide is 3-8% in thefirst dispersion liquid.
 4. The preparation method according to claim 1,wherein a solid content of the weak graphene oxide is 7-11% in thesecond dispersion liquid.
 5. The preparation method according to claim1, wherein the slurry is dispersed at a linear velocity of 50-300 m/s.6. The preparation method according to claim 1, wherein defoaming iscarried out using a continuous on-line defoaming machine rotating at aspeed of 2000-3000 r/min.
 7. The preparation method according to claim6, wherein the defoaming machine is operating at 400-600 Pa.
 8. Thepreparation method according to claim 1, wherein the wet film is 2-5 mmin thickness.
 9. The preparation method according to claim 1, whereincoating is carried out at a speed of 1-2 m/min.
 10. The preparationmethod according to claim 1, wherein drying is carried out at atemperature of 50-100° C. and for a duration of 0.5-1.5 h.
 11. Thepreparation method according to claim 1, wherein the high temperaturetreatment is carried out by keeping a temperature of the composite filmat 1000-3000° C. for 5-15 h.
 12. The preparation method according toclaim 11, wherein Hall the temperature of the composite film is raisedfrom room temperature to 1000-3000° C. at a heating rate of 0.2-5°C./min.
 13. The preparation method according to claim 12, wherein theheating rate is 1-1.5° C./min when the temperature of the composite filmis 100° C. or less, the heating rate is 0.2-0.5° C./min when thetemperature of the composite film is between 100° C. and 300° C., andthe heating rate is 3-5° C./min when the temperature of the compositefilm is above 300° C.
 14. The preparation method according to claim 1,wherein the compacting is carried out at a compacting pressure of 10-30MPa.